With rapid development of a modern semiconductor technology, a conventional design of an analog device faces more challenges. As a supply voltage continuously decreases, the supply voltage is to be smaller and have higher integrity. In addition, a threshold voltage of a complementary metal oxide semiconductor does not change relatively in a linear manner as a size of a transistor changes. In this way, margin space of an adjustment voltage for the conventional design of the analog device becomes smaller, especially in the supply voltage case in which power supply of the analog device is below 1 volt. An operational amplifier is one of most frequently used devices in the analog device, and also is a main device. With power supply with an ultra-low voltage, problems urgent to be resolved of the operational amplifier are a wide range of a working voltage, and stability of an output voltage.
For an ultra-low voltage bulk-driven operational amplifier, transconductance of a differential input stage is different due to changes, of a threshold voltage and an equivalent channel length of a bulk-driven transistor, caused by a technically physical factor or a noise factor in an actual working state, that is, a current that is input to an amplification stage may generate a corresponding non-linear change. Consequently, the operational amplifier is unstable. In addition, the operational amplifier works in an ultra-low voltage state, an existing rail-to-rail output circuit is very complicated, and costs are relatively high.